The invention relates to a discriminating device for linearly frequency modulated continuous wave range measuring radar having an FM/CW sawtooth frequency comprising, in a control loop of the frequency fb of the subtractive beat signal Fb between transmitted waves and waves received by way of echoes, a pursuit discriminator for maintaining the frequency fb at a fixed value f.sub.0, and including a pursuit validation circuit.
The invention particularly applies to radio altimeters on board aircraft or to proximity fuses for missiles. When applied to radio altimeters it intends to avoid the latter undesirably locking on to echoes from rain or hail and providing faulty altitude indications.
As regards the proximity fuses, the recognition of a Volume-Scattered Echo (VSE in the sequel of the text) allows of inhibiting the operation of the fuses in the case of rain, hail or particles and thus avoiding undesired triggerings.
Irrespective of their type of modulation, pulse or FM/CW modulation, the radars can detect scattered, volume-distributed echoes and this can be performed more easily according as the wavelength used is shorter. This property is advantageously used in meteorological radars to detect stormy precipitation. In other types of radar such as, for example, aircraft radio altimeters, the detection of precipitation located between the aircraft and the ground forms a serious drawback. One alleviates this drawback mainly by using sufficiently large wavelengths, for example: .lambda.=7 cm for the radio altimeters instead of: .lambda.=2 cm for the meteorological radars. However, aircraft automation which has become more and more advanced particularly during the take-off phase, requires an enhanced protection against faulty indications, even brief indications, which may be caused by an undesired locking on to heavy precipitation by the radio altimeter, which does not occur very frequency though, but it not exceptional.
More particularly, French Patent Specification No. FR-A 1.557.670, corresponding to U.S. Pat. No. 3,588,899 is known to provide a radio altimeter of the type defined in the opening paragraph having internal control elements, specifically an element termed contrast discriminator, for controlling the power and the form of the frequency spectrum of the beat signal on either one of the two sides of the reference frequency f.sub.0, for example, equal to 25 kHz. In an FM/CW linear slope radar system the spectrum of the beat frequency, after reflection of the wave by a flat undefined scattering ground W.sub.S (f), has non-symmetrical shape with a maximum amplitude for a minimum frequency fmin and W.sub.S (f) then strongly diminishes to a value denoted fmax so that: EQU fmax=fmin/cos .theta.
with hypothesis of a concical radiation lobe of half the angle having a width of .theta.. Beyond fmax the amplitude of the spectrum becomes negligible.
The frequency fmin corresponds with the minimum range, thus with the altitude and the significant part of the spectrum is concentrated in a frequency band ranging from fmin to fmax, termed centre band which does not exceed 15% as a relative value in the practical case in which .theta..perspectiveto.30.degree., that is to say: ##EQU1##
When the altimeter loop is in the lock-on search phase (when switched on, after an unlocking or during a sequential search cycle for verification of the integrity), the spectrum which may have been reflected by the ground scans the frequency axis by passing at the right of the reference frequency f.sub.0. The contrast discriminator, centred at f.sub.0, permanently compares the energy in a centre band window having the frequency f.sub.0 with the out-of-band energy (one or two adjacent sideband windows), the frequency characteristic of the discriminator being arranged in a manner such that a flat white noise appreciably balances the weighing referenced C, the power received in the centre band window being counted positive and that received in the sideband windows being counted negative.
In the presence of the signal reflected by the ground, the spectrum of Fb is almost entirely contained in the centre band window, and the balance of the weighing exceeds a predetermined positive threshold C.sub.0, which entails that the altimeter pursuit loop is maintained to be locked on to the ground.
For a volume-scattered echo of precipitation the form of the spectrum of Fb is different from that of the ground echo indicated above: the maximum of the spectrum always being adjusted substantially to the frequency f.sub.0, part of the spectrum being situated in the sideband window of the lower frequencies, in the preferred case where there are two sideband windows, the envelope being a curve having steep positive slopes, and after passing the maximum in the centre band window, the envelope of the spectrum being subjected to slow decrease so that non-negligible powers are present in the whole sideband window of the higher frequencies and even beyond them if the depth of the volume of precipitation is sufficient, which is generally the case. Experinece and computations have shown that for echoes of considerable precipitation, the weighing effeted by the contrast discriminator may exceed the threshold C.sub.0, thus authorising the locking on of the radio altimeter to a range situated inside the volume of precipitation. Such an erroneous locking on occurs when the following inequality is realised: ##EQU2## W.sub.V (f): Spectrum of Fb (power), H(f): Weighting function of the windows of the contrast discriminator,
C.sub.0 (h): Positive threshold as a function of the altitude shown according to an internal law of the system; this law reproduces approximately the theoretical space attenuation between the transmit aerial and the receive aerial after absorption and reflection by the ground which is nominally the least reflecting (.sigma..sub.0 min=-23 dB). PA1 The reference to the ratio C/D and its use are not very sensitive to specifically the rolling motion of the aircraft support, nor to the statistical fluctuations of the precipitation to be expected and to the dynamics of the receiver. PA1 The variation of the ratio C/D as a function of the altitude and taking into account the spectral degradation at low altitudes shows, when putting the decision threshold with respect to the variation of C/D at +3 dB, that: PA1 for a cos.sup.4 .theta. narrow radiation pattern, the useful range of the invention extends from 10 feet to 5000 feet for support variations of .+-.30.degree., and maximum precipitation (.eta.=-30 dB), PA1 for a cos.sup.0,5 .theta. wide radiation pattern, the useful range extends from 10 feet to 15000 feet for support variations of .+-.60.degree. (rolling) and maximum precipitation (.eta.=-30 dB).
Another factor presenting itself in the technical problem of a radio altimeter locking on to a VSE is the radiation law of the aerial (rolling diagram). Since this law is of the cos.sup.n .theta. type, the value of n conditions the influence of the rolling motion on the altitude measurement and also, as shown by the computation, the sensitivity to the locking on to precipitation.
In order to resolve the technical problem posed one has thought of optimising the contrast discriminator so that it can also perform, among other things, a discrimination of a volume-scattered echo for a given form of its spectrum that is sufficiently different from that of a ground echo. This optimisation is possible and permits a certain rejection of the VSE's for the value of .theta. exceeding 30.degree.. However, the radiation angles of the aerials imposed on the radio altirmeters are typically comprised between 20.degree. and 30.degree. and the optimised contrast discriminator is thus not enough to sufficiently refect the VSE's, especially because of the very strong dynamics of the signals linked with the external parameters, that is to say, with the types of precipitation that can be expected. In effect, the reflection coefficient .eta. in m.sup.2 /m.sup.3, that is to say, the surface equivalent radar (SER) per precipitation volume, expressed in dB (10 log..eta.) ranges from -54 dB for heavy rain (100 mm/h), to -42 dB for dry hail and -32 dB for wet hail, the latter representing the strongest type of precipitation.